In the majority of screen printing operations, more than one color is used to create the desired image on a substrate, such as a shirt or poster. The use of more than one color results in using several screens, one for each color, wherein each screen has the associated image for that color. It is therefore necessary to ensure that the images from each screen are aligned properly with each other, so that each associated color is aligned properly on the substrate, as quickly as possible for economic reasons.
The process consist of the final image being divided up into multiple images which are each a single color. These images can be half-tones (i.e, dots), lines, or areas. A film containing the appropriate image is produced for each specific color. In some instances, two image films can be produced for a single color. For example, when it is desired to place a color as a background and then use the same color for a detail on top of an intermediate printed color, two image films would be used.
Once the image films are ready, a screen mesh is prepared. The screen mesh is held at a proper tension in a screen printing frame and carries a light sensitive coating on film. The screen mesh is placed in contact with the image film on an exposure table. The films are placed in relative alignment and the images are burned into (transferred to) the light sensitive coating on the screen priming frame. The exposure table has a single source light, such as carbon metal hallide arc or a mercury vapor light, to transfer the image.
In order to ensure that the films do not move relative to each other during processing, the exposure table has a vacuum blanket overlying the glass table. A vacuum created between the glass table and the vacuum blanket pulls the film having the image into complete intimate contact with the screen mesh containing the light sensitive emulsion or coating. After the image is burned into the emulsion or coating on the screen mesh, the screen mesh is washed with water to remove the unexposed coating to make the image on the screen mesh relatively permanent.
The screen printing frames, each containing a screen mesh having an image, are placed in a printing machine having several stations; one station for each image. Several test prints are taken to determine if the printed image from each color/screen is aligned. The images of the multiple screen printing frames are adjusted by moving the screen printing frames relative to the printing machine so that the multiple colors are aligned properly in the printing machine to achieve The desired image.
It has been recognized that the more consistent the alignment of each image on the screen mesh in each of the screen printing frames, the less time and effort that is typically required to precisely align the screen printing frames within the printing machine to arrive at production printing.
One prior method of ensuring consistent positioning of the images on the screen fabrics of the screen printing frames for a screen printing job having numerous colors, involves placing the various images 20 on various image films 22 and aligning the various image films 22 on a lay-up board 24. As seen in FIG. 1, the lay-up board 24 has a registration bar 26 secured to one end. The registration bar 26 has a plate 28 from which a set of pins 30 project upward for receiving a plurality of holes 32 in the image film 22 containing an image.
The lay-up board 24 has a series of lines 34 for assisting in the lay-up. The various images (at least one for each color) are placed on a film in a photographic process which is well-known in the art. The film is either punched with the holes or aligned and secured on a carrier sheet having the holes to create the image films 22. After all the images 20 on the various image films 22 are aligned on the lay-up board 24 and have the punched holes, the image 20 on each image film 22 needs to be transferred to a screen printing film 40 carried by a screen mesh 38 of a screen printing frame 42.
Referring to FIG. 4, a pin registration bar 44 is secured to a glass table 46 of an exposure table 48 using a pair of suction cups 50, as seen in FIG. 2A. The pin registration bar 44 has two sets of pins 52 and 54 projecting upward from an upper surface 56 of a plate 58. One set of upwardly projecting pins 52 is adapted to be received by the holes 32 located in the image film 22. Referring to FIG. 2, the second set of pins 54 are located outboard of the first set of pins 52 and are symmetric about a center line "C".
Referring to FIG. 3, the pin registration bar 44 is shown in perspective. The first set of pins 52 project upward from the upper surface 56 of the plate 58. The series of pins 52, which are symmetric about the center line "C", have a plurality of square shaped pins 52a and one circular shaped pin 52b located at the center line "C" of the pin registration bar 44. The second set of pins 54, which likewise project upward from the upper surface 56 of the plate 58, are spaced outward of the first set of pins 52.
Referring back to FIG. 4, the image film 22 for a specific color and an associated screen printing frame 42 is laid on the glass table 46 with the holes 32 of the image film 22 receiving the first set of pins 52 on the pin registration bar 44. In that the image film 22 has both circular and square holes 32, the image film 22 cannot be mis-aligned.
With the image film 22 located on the glass table 46, the screen printing frame 42, which has the screen printing film 40 on the screen mesh 38, is positioned such that a pair of openings 62 carried by the screen printing frame receives two of the second set of pins 52, as seen in FIG. 2. The opening 62, such as a hole or a slot, is located in a comer member 64 or associated parts of the screen printing frame 42, such as a registration adapter 66 as seen in FIG. 2. The openings 62 on the screen printing frame 42 are positioned such that those pins 54 which project upward from the plate 58 of the pin registration bar 44 and are not received by the openings 62, do not interfere with the tensioning rollers 68 of the screen printing frame 42. The pins 54 are located closer to the bar portion of the pin registration bar 26 than the longitudinal axis of the tensioning roller 68.
A vacuum blanket 72 of the exposure table 48 is lowered onto the screen printing frame 42 and the glass table 46 and a vacuum is drawn to ensure that there no movement between the films 22 and 40. The exposure table 48 has a light source, not shown, which underlies the glass table 46 for exposing the screen printing film 42 located on the screen mesh 38 of the screen printing frame 40. The high intensity light bums the image 20 located on the image film 22 into the screen printing film 40 by sending light through the image film 22 onto the screen printing film 40, at specific locations, therein transferring the image 20 from the image film 22 to the screen printing film 40. With the image 22 on the screen mesh 38 of the screen printing frame 42, the screen mesh 38 including the screen printing film 40 is treated so that the screen printing film 40 is ready to print.
Referring to FIG. 5, with the image 22 properly aligned and treated on the screen printing film 40 carried by the screen mesh 38 of the screen printing frame 42, the screen printing frame 42 is placed in a "U" shaped channel 74 of a screen printing machine. The "U" shaped channel 74 has a plate 78 through which a series of pins 80 project upward, only one seen. The "U" shaped channel 74 with the plate 78 and pins 80 acts as a registration apparatus 82. U.S. Pat. No. 5,377,422 discloses such a pin registration system for screen printing frames, and is incorporated herein by reference. The series of pins 80 are positioned similar to the second set of pins 54 on the pin registration bar 44. The hole (opening) 62 of the registration adapter 66 of the screen printing frame 42 receives one of the pins 80 located in the plate 78 to position the screen printing frame 42 relative to the registration apparatus 82 of the screen printing machine. The other registration adapter 66 of the screen printing frame 42, associated with another comer member 64, has a slot as the opening to assist in the alignment by preventing rotation of the screen printing frame 42 about the pins 80/hole 62 connection.
With the screen printing frame 42 properly aligned in the screen printing machine, the screen printing frame 42 moves relative to the substrate to move the screen mesh 38 in proximity to the substrate, not shown, allowing the transfer of the image from the screen printing film 40 to the article. In that the image 22 has been aligned precisely from the image film 22 to the screen printing film 40 on the screen printing frame 42, and the screen printing frame 42 has been mounted precisely on the printing machine, the images from the multiple screen printing frames 42 on the printing machine should align precisely on the substrate. Any alignment of the screen printing frames 42 required within the machine should be minor to perfect the image.
One of the shortcomings of this method is that this pin registration bar cannot be used with an exposure table which has the vacuum blanket located under the glass table during alignment, referred to as a glass-up exposure table. In addition, lateral forces which might develop on the screen printing frame, the pin registration bar, and the image film, when a vacuum is being drawn on the exposure table could result in the screen printing frame and the image sheet shifting relative to each other, at the end not engaging the registration bar. If the screen printing frame shifts relative to the image film, the shift on each screen printing frame/image film would likely not be the same, for each of the colors, therein resulting in non-proper alignment for multiple color final image.
It is desired to have a means of ensuring alignment of the image films to the film on the screen mesh during exposure for a glass-up vacuum table and to ensure that lateral forces do not create shifting of the image. In addition, it is desired to be able to use a single apparatus to produce multiple screen printing frames on a single exposure table at one time.